The field of this invention relates to methods and apparatus for an envelope tracking system, and in particular to methods and apparatus for improving an efficiency and linearity of an envelope tracking system for a power amplifier module, for example within a radio frequency (RF) transmitter module of a wireless communication unit.
A primary focus and application of the present invention is the field of radio frequency (RF) power amplifiers capable of use in wireless telecommunication applications. Continuing pressure on the limited spectrum available for radio communication systems is forcing the development of spectrally-efficient linear modulation schemes. Since the envelopes of a number of these linear modulation schemes fluctuate, these result in the average power delivered to the antenna being significantly lower than the maximum power, leading to poor efficiency of the power amplifier. Specifically, in this field, there has been a significant amount of research effort in developing high efficiency topologies capable of providing high performances in the ‘back-off’ (linear) region of the power amplifier.
Linear modulation schemes require linear amplification of the modulated signal in order to minimise undesired out-of-band emissions from spectral re-growth. However, the active devices used within a typical RF amplifying device are inherently non-linear by nature. Only when a small portion of the consumed DC power is transformed into RF power, can the transfer function of the amplifying device be approximated by a straight line, i.e. as in an ideal linear amplifier case. This mode of operation provides a low efficiency of DC to RF power conversion, which is unacceptable for portable (subscriber) wireless communication units. Furthermore, the low efficiency is also recognised as being problematic for the base stations.
Additionally, the emphasis in portable (subscriber) equipment is to increase battery life. To achieve both linearity and efficiency, so called linearisation techniques are used to improve the linearity of the more efficient amplifier classes, for example class ‘AB’, ‘B’ or ‘C’ amplifiers. A number and variety of linearising techniques exist, which are often used in designing linear transmitters, such as Cartesian Feedback, Feed-forward, and Adaptive Pre-distortion.
Voltages at the output of the linear, e.g. Class AB, amplifier are typically set by the requirements of the final RF power amplifier (PA) device. Generally, the minimum voltage of the PA is significantly larger than that required by the output devices of the Class AB amplifier. Hence, they are not the most efficient of amplification techniques. The efficiency of the transmitter (primarily the PA) is determined by the voltage across the output devices, as well as any excess voltage across any pull-down device components due to the minimum supply voltage (Vmin) requirement of the PA.
In order to increase the bit rate used in transmit uplink communication channels, larger constellation modulation schemes, with an amplitude modulation (AM) component are being investigated and, indeed, becoming required. These modulation schemes, such as sixteen quadrature amplitude modulation (16-QAM), require linear PAs and are associated with high ‘crest’ factors (i.e. a degree of fluctuation) of the modulation envelope waveform. This is in contrast to the previously often-used constant envelope modulation schemes and can result in significant reduction in power efficiency and linearity.
To help overcome such efficiency and linearity issues a number of solutions have been proposed.
One known technique, as illustrated in the block diagram 100 of FIG. 1, relates to controlling the supply voltage 120 to the power amplifier 140. This technique is known as average power tracking (APT). With APT, an average power level 105 of the transmitted signal is determined and applied to an APT-Vpa mapping module 110 that determines a supply voltage (Vpa) 120 to be applied to the PA 140 based on the determined average power level. This signal is then applied to a DC-DC converter 115 and the resultant (output) voltage is applied to the Power Amplifier 140 as its supply voltage (Vpa) 120. In such APT techniques, there is always a substantially fixed load 145 for the PA 140 prior to radiating the transmit signal from an antenna (not shown). One known problem with this technique is that it operates with less efficiency at higher output power when the peak to average power ratio (PAPR) back-off is large, which is predominantly the case for more complex modulation schemes.
Another known supply voltage technique 200 is envelope tracking (ET), illustrated in FIG. 2, which relates to modulating the radio frequency (RF) power amplifier (PA) supply voltage (Vpa) 220 to match (e.g. track) the envelope of the radio frequency waveform being transmitted by the RF PA 240. Typically, ET systems control the RF PA supply voltage 220 in order to improve PA efficiency through selecting a lower supply voltage dependent upon an instantaneous envelope of the input signal, and to improve linearity by selecting a RF PA supply voltage 220 dependent upon a constant PA amplification gain. A digital (quadrature) input signal 202 is input to an RF transmitter 230, whose output provides an input power level 235 to the RF PA 240. The RF PA output 225 is typically output to a fixed load 245. Concurrently, the digital (quadrature) input signal 202 is applied to an envelope detector 204 arranged to determine a real-time envelope of the signal to be transmitted (e.g. radiated). The determined real-time envelope signal output from the envelope detector 204 is input to an envelope mapping function 210, which is arranged to determine a suitable PA supply voltage (Vpa) 220 to be applied to the PA 240 to substantially match the instantaneous real-time envelope of the signal to be transmitted. The output from the envelope mapping function 210 is input to a delay control function 212 that aligns, in time, the PA supply voltage (Vpa) 220 to the signal being passed through RF transmitter 230. The output from the delay control function 212 is input to a supply modulator 214 that provides the PA supply voltage (Vpa) 220 to be applied to the PA 240.
With ET, the instantaneous PA supply voltage (Vpa) 220 of the wireless transmitter is caused to approximately track the instantaneous envelope (ENV) of the transmitted RF signal. Thus, since the power dissipation in the PA 240 is proportional to the difference between its supply voltage and output voltage, ET may provide an increase in PA efficiency, reduced heat dissipation, improved linearity and increased maximum output power 225, whilst allowing the PA to produce the intended RF output. However, the total system efficiency is affected by supply modulator efficiency that is related to the supply modulator design, supply voltage range, bandwidth and PA loading, which typically results in ET modulator efficiency not being high enough for most applications. The envelope mapping function 210 between ENV and VPA is critical for optimum performance (efficiency, gain, and adjacent channel power (ACP)). Also critical to system performance is timing alignment between the RF signal and VPA at the PA.
A yet further known technique 300 is to combine envelope tracking (ET) with digital pre-distortion (DPD) and combine this with dynamic load modulation (DLM), as illustrated in FIG. 3. Here, control/manipulation of the input waveform/signal in the digital domain is performed in order to compensate for PA nonlinearity (AM-to-AM and AM-to-PM) effects, thereby improving PA output linearity based on prior information or measured data of the PA system. A tunable matching network (TMN), or Variable matching network (VMN), is implemented to provide a variable impedance network to PA output loading as part of DLM.
Again, a digital (quadrature) input signal 302 is input to an RF transmitter 330 via a digital pre-distortion (DPD) function 326, whose output provides an input power level 335 to the RF PA 340, driven by supply voltage Vdc 320. The RF PA output 325 is output to a tunable matching load 345. Concurrently, the digital (quadrature) input signal 302 is applied to an envelope detector 304 arranged to determine a real-time envelope of the signal to be transmitted (e.g. radiated). The determined real-time envelope signal output from the envelope detector 304 is input to an envelope mapping function 310, which is arranged to determine a suitable control voltage (Vc) 316 to be applied to the tunable matching load 345 to substantially ensure the maximum PA efficiency in the transmission. The output from the envelope mapping function 310 is input to a delay control function 312 that aligns, in time, the control voltage (Vc) 316 to the signal being output from the PA 340. The output from the delay control function 312 is input to an operational amplifier 314 that provides the control voltage (Vc) 316 to be applied to the tunable matching load 345. In this manner, PA load control may be achieved by adjusting PA output load impedance in the tunable matching load 345 to correspond to an average output power or to correspond to the envelope of input signal.
However, there is a need to use (voltage) operational amplifier 314 in order to provide higher control voltage for varactors located in the tunable matching load 345. Furthermore, this approach may induce TX signal bandwidth re-growth due to maximum-efficiency DLM mapping with DPD.
In this manner, envelope-tracking can be combined with digital pre-distortion (DPD) on the RF signal to improve ACP robustness. However, since the ET system is often a multichip implementation involving function blocks in digital baseband (BB), analogue BB, RF transceiver, power management and PA, consistent ET system performance cannot easily be guaranteed across all devices by hardware.
A yet further technique is described in US008093945 B2, titled ‘Joint supply and bias modulation’ (by Nujira and published in 2012), whereby the supply and bias voltages are adjusted according to instant envelope mapping.
Thus, there is a need for a more efficient and cost effective solution to the problem of improving PA efficiency and ET linearity.